Electrical connection to a micro electro-mechanical system

ABSTRACT

A MEMS device includes, in part, first and second conductive semiconductor substrates, an insulating material disposed between the semiconductor substrates, a cavity formed in the second semiconductor substrate, and at least first and second drive masses each of which includes a multitude of beams etched from the first semiconductor substrate and is adapted to move in the cavity in response to an applied force. At least a first portion of the first substrate is adapted to move in response to the applied force and causes the at least first and second drive mass to be in electrical communication with the first substrate. The device may further include, in part, a coupling spring disposed between and in electrical communication with the first and second drive masses. The coupling spring is adapted to provide electrical communication between a second portion of the first substrate and the first and second drive masses.

FIELD OF THE INVENTION

The present invention relates to Micro Electro-Mechanical Systems(MEMS), and more particularly to electrical connectivity in MEMS.

BACKGROUND

MEMS, such as motion sensors, inertial sensors, and movable mirrors, arebeing widely used. As is well known, a MEMS motion sensor may be, forexample, an accelerometer for detecting linear motion, or a gyroscopefor detecting rotation and angular velocities. Advanced planar siliconmanufacturing processes have become the main manufacturing techniques inMEMS. However, a need continues to exist for improvement inmanufacturing MEMS devices.

BRIEF SUMMARY OF THE INVENTION

A MEMS device, in accordance with one embodiment of the presentinvention, includes, in part, a first conductive semiconductorsubstrate, a second conductive semiconductor substrate, an insulatingmaterial disposed between the first and second conductive semiconductorsubstrates, a cavity formed in the second conductive semiconductorsubstrate, and at least first and second drive masses each of whichincludes a multitude of beams etched from the first conductivesemiconductor substrate and adapted to move in the cavity and inresponse to an applied force. At least a first portion of the firstconductive semiconductor substrate is adapted to move in response to theapplied force and causes the at least first and second drive mass to bein electrical communication with the first conductive semiconductorsubstrate.

In one embodiment, the MEMS further includes, in part, at least onecoupling spring disposed between and in electrical communication withthe first and second drive masses. The coupling spring is adapted toprovide electrical communication between a second portion of the firstconductive substrate and the first and second drive masses. The secondportion of the first conductive semiconductor substrate is adapted notto move in response to the applied force

In one embodiment, the first and second semiconductor substrates aresilicon substrates and the first insulating layer is a silicon oxidelayer. In one embodiment, the MEMS device further includes, in part, anisolation joint formed in the first conductive semiconductor substrate.

In one embodiment, the MEMS device further includes, in part at leastone spring isolated from at least a portion of each of the first andsecond drive masses by the isolation joint. In one embodiment, each beamis a silicon beam. In one embodiment, the MEMS device further includes,in part, a metal layer formed on a surface of the first semiconductorsubstrate, and a wirebond that is in electrical communication with thesecond portion of the first semi-conductor substrate via the metallayer.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a cross-sectional view of a silicon-on-insulator wafer(substrate) in which a MEMS device is formed, in accordance with oneembodiment of the present invention.

FIG. 2 is a cross-sectional view of the substrate of FIG. 1 following athermal oxidation process, in accordance with one exemplary embodimentof the present invention.

FIG. 3 is a cross-sectional view of the substrate of FIG. 2 followingthe formation of an opening in the oxide, in accordance with oneexemplary embodiment of the present invention.

FIG. 4 is a cross-sectional view of the substrate of FIG. 3 followingthe formation of a trench, in accordance with one exemplary embodimentof the present invention.

FIG. 5 is a cross-sectional view of the substrate of FIG. 4 followingthe filling of the trench, in accordance with one exemplary embodimentof the present invention.

FIG. 6 is a cross-sectional view of the substrate of FIG. 5 followingthe formation of an opening in the oxide and deposition of a layer ofscreen oxide, in accordance with one exemplary embodiment of the presentinvention.

FIG. 7 is a cross-sectional view of the substrate of FIG. 6 following adopant implantation step and removal of the screen oxide, in accordancewith one exemplary embodiment of the present invention.

FIG. 8 is a cross-sectional view of the substrate of FIG. 7 followingthe deposition and patterning of a first metal layer, in accordance withone exemplary embodiment of the present invention.

FIG. 9 is a cross-sectional view of the substrate of FIG. 8 following anumber of deposition and patterning steps, in accordance with oneexemplary embodiment of the present invention.

FIG. 10 is a cross-sectional view of the substrate of FIG. 9 followingthe deposition of a second passivation layer, in accordance with oneexemplary embodiment of the present invention.

FIG. 11 is a cross-sectional view of the substrate of FIG. 10 followinga number of etching steps, in accordance with one exemplary embodimentof the present invention.

FIG. 12 is a cross-sectional view of the substrate of FIG. 11 followingdeposition and etching steps, in accordance with one exemplaryembodiment of the present invention.

FIG. 13 is a cross-sectional view of the substrate of FIG. 12 followingthe formation of cavities in the substrate's handle wafer and a numberof etching steps, in accordance with one exemplary embodiment of thepresent invention.

FIG. 14 is a cross-sectional view of the substrate of FIG. 13 followingthe formation of a wirebond, in accordance with one exemplary embodimentof the present invention.

FIG. 15A is a simplified high-level top view of a MEMS device of FIG.14, in accordance with one exemplary embodiment of the presentinvention.

FIG. 15B is a simplified expanded view of a portion of the MEMS deviceof FIG. 15A, in accordance with one embodiment of the present invention,

DETAILED DESCRIPTION OF THE INVENTION

In accordance with one embodiment of the present invention, a MEMSdevice includes, in part, an electrical connection to the drive massesthat is externally driven. FIG. 1 is a cross-sectional view of a silicondevice layer (also referred to herein alternatively as wafer) 100 and asilicon handle wafer 200 with a buried oxide layer 150 disposedtherebetween. Device layer 100, handle wafer 200 and the buried oxidelayer 150 collectively form a silicon-on-insulator (SOI) substrate orwafer.

To form a MEMS, in accordance with one embodiment of the presentinvention, as shown in FIG. 2, during a thermal oxidation process, alayer 310 of thermal oxide is grown on the top and back sides of the SOIwafer. Next, using a patterning and reactive ion etching (RIE) steps, anopening 315 is formed in oxide layer 310, as shown in FIG. 3.

Thereafter, using a DRIE process, a trench 325 is formed in device layer100 at opening 315 which extends to and stops on oxide layer 150, asshown in FIG. 4. Trench 325 is then filled with oxide to form isolationjoint 335, as shown in FIG. 5. Subsequently, oxide layer 310 isplanarized. In one embodiment, the thickness of oxide layer 310following the planarization step is about 1 μm. Although the Figures areshown as including one such isolation joint 335, it is understood thatother embodiments may include more than one isolation joint.

Next, a contact opening 350 is formed in oxide layer 310 of device layer100 (using, for example, fluorine based RIE) and a relatively thin layerof screen oxide 345 is grown on the resulting structure, as shown inFIG. 6. The thickness of screen oxide layer 345 is often less than thethickness of oxide layer 310. For example, in one embodiment, when oxidelayer 310 has a thickness of 1.1 μm, screen oxide layer 345 may have athickness of 25 nm. It is understood that the various layers andstructures shown in the Figures are not drawn to scale. Thereafter, adopant, such as Boron ions, are implanted through screen oxide layer345, thereby ensuring electrical contact between the first metal layerand the region near the top surface of the device layer 100

Next, a sacrificial oxide etching step is performed using, for example,buffered oxide etching step (BOE), thereby removing oxide layer 345, asshown in FIG. 7. Thereafter, a layer of metal, such as Aluminum, isdeposited, using for example physical vapor deposition (PVD), andpatterned so as to form metal trace 360 which also forms a contact withthe Boron-doped silicon 100 positioned below opening 350, as shown inFIG. 8. In one embodiment, metal trace 360 has a thickness ranging from0.1 um to 0.5 um.

To form a second metal layer, a second layer of passivation 400, such asTEOS oxide, is deposited on the device structure shown in FIG. 8 andsubsequently patterned. In one embodiment, such a passivation layer mayhave a thickness of 1.0 μm. Thereafter, a second layer of metal 410 isdeposited and patterned, using for example, spray etching. In oneembodiment, Aluminum is used as a second layer metal having a thicknessof, e.g., 0.7 μm. FIG. 9 shows the resulting device structure after thepatterning of the second metal layer. As shown in FIG. 9, a portion ofsecond metal layer 410 is seen as forming an electrical contact with aportion of first metal layer 360.

Next, as shown in FIG. 10, another passivation layer 450, such as TEOSoxide, that may have a thickness of e.g. 0.2 μm, is deposited over thedevice structure of FIG. 9. A sintering bake process in the temperaturerange from 375 C to 450 C may be used to enhance electrical connectionbetween the first layer of metal and the top silicon surface in thecontact opening area. As is well known, a rapid thermal anneal process(RTA) may also be used to achieve the same effect in forming themetal-silicon contact. Although not shown, additional layers of metal(such as third and fourth metal layer) may be deposited, patterned andpassivated in a manner similar to those described above with respect tothe first and second metal layers.

Next, using a standard lithography process, a multitude of beam 800(four of which are shown in the exemplary embodiment of FIG. 11) areformed, as described below. To form the beams, in one embodiment, afluorine based RIE etching process is used to etch the passivationlayer. Thereafter, using a DRIE etching process, any silicon exposed indevice 100 wafer is etched anistropically. Next, using, for example, afluorine based RIE etching process, any exposed silicon oxide layer 150(also referred to herein as buried oxide layer) is also etched.Thereafter, the photoresist is also removed thereby resulting in theformation of a multitude of beams 800 each of which is shown asincluding a bottom layer formed from oxide 150, a middle layer formedfrom silicon 100, and a top layer formed from passivation layer 450, asshown in FIG. 11. Beams 800 are components of the drive masses of theMEMS.

Next, a layer of TEOS oxide is deposited using, for example, PECVDtechnique, along sidewalls 700 of beams 800. The deposited TEOS oxide700 covers any exposed silicon surfaces that have not already beencovered with passivation layers. Any oxide layer covering the topsurface of handle wafer 200 is subsequently removed using a fluorinebased RIE step, as shown in FIG. 12. Thereafter, using a DRIE etchingprocess, silicon is etched isotropically from handle wafer 200 to formcavity 730 as shown in FIG. 13. Cavity 730 enables the free movement ofbeams 800 of the MEMS drive masses in response to an applied force.

Next, a vapor HF etch step is performed to remove any exposed oxide andpassivation layers on the moving MEMS structures, thereby to from thedevice structure shown in FIG. 13. Each beam 800 shown in FIG. 13 thusincludes only silicon. Furthermore, a portion of oxide 150 in cavity 730is also removed. Next, a wirebond 180 is formed on metal layer 410, asshown in FIG. 14. Wirebond 810 thus provides an external connection tometal layer 410 and thus to the underlaying device layer 100 which hasbeen implanted/doped with Boron and therefore has a relatively highconductivity.

In one embodiment, beams 800 of the drive masses form an electricalcontact with metal layer 410 through the underlaying device layer 100and out of the plane of FIG. 14. Therefore, in such embodiments,electrical connection is made between wirebond 810 and beams 800 of thedrive masses via metal layer 410, metal layer 360 and region 840 ofdevice layer 100.

FIG. 15A is a simplified high-level top view of a MEMS device 900, inaccordance with one exemplary embodiment of the present invention. MEMSdevice 900 is shown as including, in part, a pair of drive masses 850that are coupled to one another via a coupling spring 860. Each drivemass 850, which includes, in part, a multitude of beams 800 (see, forexample, FIG. 14) is also shown as being coupled to springs 870.Wirebond 810 is formed on metal trace 410 that is in electrical contactwith underlaying device layer 100 though metal trace 360 and via 350, asdescribed with reference, for example, to FIG. 14. Also shown in FIG.15A are etched regions 890 of device layer 100.

Drive masses 850 and coupling spring 860 are both formed from devicelayer 100 which, as described above, is highly conductive due to theimplantation process. Accordingly, drive masses 850 and coupling spring860 are in electrical communication with one another. Drive masses 850and coupling spring 860 are in further electrical communication withwirebond 810 through metal traces 410 and 360 via region 840 of devicelayer 100, as was also described above. FIG. 15B is an expanded view ofregion 880 of FIG. 15A showing portions of etched regions 890 andcoupling spring 860 as well as region 840 of device layer 100. FIG. 14is a cross-sectional view of MEMS device 900 when viewed along linesA-A′. It is understood that the connection between region 840 of devicelayer 100 and beams 800 is made out of the plane of FIG. 14. Regions 840of device layer 100, which move in response to an applied force, are inelectrical contact with first layer metal 360 through opening 350 andhence in contact with second layer metal 410 and wirebond 810.

Although not shown, in one embodiment, springs 870, which are alsoformed in device layer 100, are electrically isolated from portions orall of the drive masses via one or more isolation joints formed indevice layer 100, such as isolation joint 335 shown in the drawings.Regions 840, which move in response to an applied force, thuselectrically connect the drive masses to the non-moving portions ofdevice layer 100. Furthermore, although FIGS. 15A and 15B are shown asincluding two drive masses, it is understood that a MEMS device, inaccordance with embodiments of the present invention, may have anynumber of drive masses, such as 4.

The above embodiments of the present invention are illustrative and notlimitative. Embodiments of the present invention are not limited by thetype of MEMS device. Embodiments of the present invention are notlimited by the type of deposition, patterning, etching, and othersemiconductor processing steps required to form the various layers andstructures described herein. Embodiments of the present invention arenot limited to any specific thicknesses of the layers described herein.Embodiments of the present invention are not limited to thematerials/layers described above. Accordingly, it is understood thatother semiconductor materials may be present between the various layersdescribed above. Other additions, subtractions or modifications areobvious in view of the present disclosure and are intended to fallwithin the scope of the appended claims.

What is claimed is:
 1. A MEMS device comprising: a first electricallyconductive semiconductor substrate; a second electrically conductivesemiconductor substrate; an insulating material disposed between thefirst and second conductive semiconductor substrates; a cavity formed inthe second conductive semiconductor substrate, said second conductivesemiconductor substrate extending along a bottom of the cavity; and atleast first and second drive masses each comprising a plurality of beamsetched from the first conductive semiconductor substrate and adapted tomove in the cavity and in response to an applied force, wherein at leasta first portion of the first conductive semiconductor substrate isadapted to move in response to the applied force and causes the at leastfirst and second drive mass to be in electrical communication with thefirst conductive semiconductor substrate.
 2. The MEMS device of claim 1further comprising: at least one coupling spring disposed between and inelectrical communication with the at least first and second drivemasses, said at least one coupling spring adapted to provide electricalcommunication between a second portion of the first conductive substrateand the first and second drive masses, said second portion of the firstconductive semiconductor substrate adapted not to move in response tothe applied force.
 3. The MEMS device of claim 1 wherein said first andsecond semiconductor substrates are silicon substrates and the firstinsulating layer is a silicon oxide layer.
 4. The MEMS device of claim 1further comprising an isolation joint formed in the first conductivesemiconductor substrate.
 5. The MEMS device of claim 4 furthercomprising at least one spring isolated from at least a portion of eachof the first and second drive masses by the isolation joint.
 6. The MEMSdevice of claim 1 wherein each of the plurality of beams is a siliconbeam.
 7. The MEMS device of claim 1 further comprising: a metal layerformed on a surface of the first semiconductor substrate; and a wirebondin electrical communication with the second portion of the firstsemiconductor substrate via the metal layer.